As shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. The terms base station and a cell may be used interchangeably herein. A cell from which a client terminal has already received service may be referred to as having visited that cell. In general, the cells that are in the vicinity of the serving cell are called neighbor cells. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell. Each base station may be identified by a unique identifier referred to herein as Cell Identity (CID). The CID of a base station may be known to a client terminal when it decodes broadcast information from the base station. The CID may be of different types such as Cell Global Identity (CGI) in case of 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system. A base station may have additional identities such as Closed Subscriber Group (CSG) identity. Furthermore, there may be an identity for the entire network to uniquely identify a particular network operator. For example, in case of 3GPP LTE wireless communication system, a Public Land Mobile Network (PLMN) identity may be used. The various identities used by a cell and the network may be broadcast by the cell as system information.
Location determination is a commonly available and used capability in many client terminals in a wireless communication system. The terms location determination and positioning are used interchangeably herein. Client terminals may obtain their own location through different technologies. A satellite navigation system with global coverage commonly known as Global Navigation Satellite System (GNSS) and the Observed Time Difference of Arrival (OTDOA) are two example technologies. Although there may be many different technologies for location determination, often the core methods used for location determination are triangulation and trilateration. Triangulation is a method of determining the location of a point by measuring angles to it from known points. The point can then be fixed as the third point of a triangle with one known side and two known angles. Trilateration is the process of determining absolute or relative locations of points by measurement of distances. In two-dimensional geometry, it is known that if a point lies on two circles corresponding to the known distances from known points, then the circle centers and the two radii (distances) provide sufficient information to narrow the possible locations down to two as shown in FIG. 2. In FIG. 2, there are two known locations La and Lb. The distance Dab between these two locations is also known based on the known locations La and Lb. The two locations L1 and L2 are equidistance from each of the two known points La and Lb. If the distances Da and Db are known, it is still not possible to uniquely determine whether the true location is L1 or L2. Additional information, such as third known point with known distance to it, may narrow the possibilities down to one unique location as shown in FIG. 3. The third location Lc and its distance Dac between the locations La and Lc and its distance Dbc between the locations Lb and Lc may be known based on known location of Lc. Furthermore, the distance Dc from the location Lc to location L1 may be known. Using these three sets of information, the coordinates of the location L1 can be uniquely determined using conventional trilateration calculations, for example, as described in “Revisiting Trilateration for Robot Localization”, by Federico Thomas and Lluís Ros, IEEE Transactions in Robotics, February 2005, pgs 93-101, incorporated by reference herein.
Location information may be obtained using other information about a cell in a wireless communication network. For example, the Round Trip Delay (RTD) or Timing Advance (TA) between a base station and a client terminal, the Received Signal Strength Indicator (RSSI), etc., combined with the known location of a cell may be used to determine the location of a client terminal. For example, the TA may be used to estimate the distance of a client terminal from the serving cell. This can narrow the location of a client terminal to a circular band of radius corresponding the distance based on TA as illustrated in FIG. 4. The timing advance typically corresponds to twice the propagation delay for a Radio Frequency (RF) signal to travel from a base station BSA to a client terminal. The TA is typically estimated by a base station when a client terminal performs initial uplink transmissions. The approximate distance between a client terminal and the BSA may be obtained by dividing half of TA by the speed of light. As illustrated in FIG. 4, there may be some uncertainty Δt in the measured value of the TA. This uncertainty may lead to uncertainty in the estimated distance using the TA. Multiple TA measurements may be used to reduce uncertainty by filtering, such as averaging. In subsequent figures, the location of a client terminal as determined based on a TA from a single base station may be illustrated as a circle instead of a circular band with uncertainty. It is to be understood that multiple TA measurements may be taken to reduce the uncertainty.
In some scenarios, it may be useful for a client terminal to know the location of one or more base stations in a wireless communication network. For example, a client terminal may make decisions about whether to make cell reselection and/or handover related measurements and system information decoding as well as how frequently to look for neighboring base stations based on the location information of the base stations.
While a network may know its location accurately, it may not provide that information to a client terminal. Furthermore, the location information for a base station may be required before a client terminal can communicate with a particular base station.